The present invention relates to a metal hydride storage facility constructed as a cylindrical pressure vessel having centrally arranged a gas (hydrogen) conduit, wherein the granulated metal capable of forming hydride is partitioned in axial direction by means of disc shaped bulk head sheets each being provided with a central opening traversed by the gas conduit.
Generally metal hydrides serve for the storage of hydrogen. A pressure vessel is usually provided and made of a metal which does not form any hydride. The particular metal hydride stored therein can be used in a variety of ways. Storage facilities of this type are based on a physical phenomenon which is comprised of a relationship between hydrogen concentration in molten metal and hydrogen pressure and temperature in the storage vessel. The development of suitable pressure vessels is, however, a significant problem. This is so because the storage facility, as stated, is comprised originally of the vessel into which a material of powdery or granular consistency has been filled. This material has the tendency of expanding physically to a considerable extent. Surprisingly the volume increase is significantly larger than expected on the basis of crystal lattice expansion based on the hydride forming process. The additional volume increase is to be contributed to the progressing subdivision and breaking up of the rather brittle grains of the storage material. Moreover it was found that in the case of high speeds the flowing hydrogen may actually redistribute the material inside of the pressure vessel. This redistribution of storage material may in fact lead locally to filling densities exceeding the maximum permissible filling density so that upon subsequently loading the material (hydrogen storage) the pressure vessel may actually experience locally pressure peaks of such high values that may lead to its destruction. Tensioning and expansion of the pressure vessel which may therefore have to be expexted as a result of excessive forces in the powder bed pose in fact a very serious problem of safety as far as operation of the device is concerned. A prevention of any progressive deforming of the vessel could lead to overdimensioning of the pressure vessel wall but that in turn leads to unacceptable overall gross weight of this storage facility. Therefore the next obvious approach appears to be limiting of the effective filling density of the material.
The reaction process of hydride storage facilities under consideration of high power and throughput during loading and unloading (hydrogen release) leaves often much to be desired. This has been contributed to the poor thermal conductivity of the powdery metal which limits technologically to a considerable extent the binding and release of enthalpy. The effect of the powder structure is a serious problem here as can be seen from the fact that technically realizable filling density of the powdery metal is actually at about 50% of the theoretical filling density.
U.S. Pat. No. 4,446,111 (see also German printed patent application No. 31 25 276) proposes to improve the heat exchange inside such a storage facility for hydrogen by means of laminalike partitioning sheets which serve as heat conductors and partition actually the hydride forming metal in axial direction in so to speak a series of discs. During the construction of the metal hydride storage facility the granular metal is preferably introduced in form of pellets of a cylindrical configuration. These pellets have a central perforation in order to accomodate the centrally disposed gas conduit running in the interior of the vessel. The purpose of providing pellets is also to be seen in that the amount of powder that is placed into the storage facility can be very accurately metered. Also, pellets are easy to handle and their use reduces fire danger. However, the production of pellets poses by and in itself another problem and is connected with a number of drawbacks. In order to make sure that pellets have the sufficient strength one has to realize that the raw material is a very brittle powderized metal. In order to ensure such strengths it is customary to ass some aluminum powder to the hydride forming powder prior to press forming it into pellets. The amount of aluminum powder so added is in the order of about 5% of the relevant volume. This Al powder acts as a binder. The addition of Al powder does indeed increase the heat conductivity of the hydride storage facility generally but only to a limited extent. The adding of Al has the drawback that for the same volume of the pressure vessel the amount of volume of hydride forming metal is correspondingly reduced, meaning that the storage capacity is somewhat reduced. Also the press working of pellets requires a fine grinding of the respective components with the grain spectrum running from 50 micrometers to about 250 micrometer. It has to be noted further that the hydride metal is very pyrophoric. This aspect requires that to a considerable extent safety features have to be provided for during the manufacture. For example, a protective gas atmosphere is mandatory which in turn makes the working even more difficult. Thus, the pellet vs. no-pellet technology constitutes a significant improvement but there is room for further improvement from an overall point of view.
It can be seen that from a technological point of view the last mentioned solution is indeed of value if the permissible filling densities are observed under adequate loading and unloading characteristic of a suitable metal hydride storage facility. Thus, an adequate use life can be expected. On the other hand the drawback is that the required production steps such as grinding, mixing and press working of the storage material into pellets is a very expensive procedure. This actually reduces the economic value and the cost of the metal hydride storage facility significantly so that the economic value of this procedure is drastically reduced. Moreover it has to be considered that the adding of aluminum powder actually prevents reuse of the molten metal for purposes of hydrogen storage because the aluminum powder simply cannot be separated anymore mechanically or through smelting from the hydride forming material; at least not in any economically feasible manner.